1. Field of the Invention
The present invention relates to antenna devices preferably for use in radio frequency identification (RFID) systems, near field communication (NFC) systems, and so on, and card devices as well as electronic apparatuses equipped with antenna devices.
2. Description of the Related Art
An antenna device is disclosed in Japanese Patent No. 4883136 as an antenna device to be mounted in a wireless communication apparatus that is applied in an RFID system or the like. The antenna device disclosed in Japanese Patent No. 4883136 is such that antenna directivity is controlled by inserting a magnetic body in an opening of an antenna coil formed on an identical plane of a flexible board.
However, the antenna device disclosed in Japanese Patent No. 4883136 has a problem as follows. That is, in the case where the number of turns is increased progressing toward an inner side portion of the winding (toward the winding center) so as to increase the total number of turns, magnetic flux that passes a coil opening is limited because an area of the coil opening becomes smaller, whereby communication characteristics thereof are deteriorated. Meanwhile, in the case where the number of turns is increased progressing toward an outer side portion of the winding while maintaining the area size of the coil opening, an outer shape of the antenna coil becomes larger.